Compressional-wave lens



W. L. HARTSFIELD COMPRESSIONAL-WAVE LENS Aug. 19, 1958 3 Sheets-Sheet 1 Filed Oct. 29, 1954 I N VEN TOR iiiaLarisfieZ ATTORNEYS Aug. 19, 1958 w. L. HARTSFIELD COMPRESSIONAL-WAVE LENS 5 Sheets-Sheet 2 Filed 061;. 29, 1954 INVENTOR IVilliwm lHa/risfie hi ATTORNEYS g- 19, 1958 w. L. HARTSFIELD 2,848,058

COMPRESSIONAL-WAVE LENS Filed Oct. 29, 1954 3 Sheets-Sheet 3 INVENTOR Wi lliamLHa/ris field ATTORNEYS COMPRESSIONAL-WAVE LENS William L. Hartsfield, Washington, l). C.

Application October 29, 1954, Serial No. 465,671

8 Claims. (Cl. 181-.5)

My invention relates to the acoustic lens art, and more particularly to improvements in parallel-plate type lens arrays for changing the distribution in space of compressional waves.

It is well known in the art that the acoustic wave fronts emitted from high frequency audio driver units are substantially plane at the upper frequencies in a direction transverse to the axis of the unit but approach a circular wave front as the frequency approaches the low frequency cut-off of the unit, and this radiation is therefore highly variable in directional characteristics, versus frequency. For instance, in the case of the driver units shown in my copending application Serial Number 411,133, filed February 18, 1954, now Patent No. 2,815,086, a listener seated directly in front of the high frequency driver unit would receive an overdose of the highs in the absence of the lens unit shown therein, but, on the other hand, a listener seated 30 degrees to one side of the axis of the driver would hear only comparatively little of the highs emitted.

It is therefore the primary object of my invention to provide an improved high frequency lens array for the purpose of controlling the distribution in space of the waves emitted by an audio transducer in such a manner that the more directional upper frequencies will be altered in distribution more than the lower frequency components which are initially emitted from the transducer with a wider dispersal in space.

More particularly, it is an object of this invention to provide a lens which will leave the vertical distribution of the input waves relatively unchanged but which will spread the waves horizontally so as to provide substantially equal distribution of the wave energy over a horizontal sector of predetermined angle. Of course, the lens may, if desired, be rotated on the front of the driver unit to spread the wave through said sector in any plane desired, other than a horizontal plane.

Still another major object of my invention is to provide a lens array which will emit a uniform distribution of wave energy over the entire width of said predetermined sector for a wide range of input frequencies, the more directional higher frequencies undergoing a greater horizontal dispersement in order to cover said sector than the lower frequencies which already have a considerable dispersement.

Another very important object of my invention is to provide a parallel-plate type of lens which is of sufficiently small physical dimensions that it will fit conveniently into conventional driver-unit cabinets, the reduction in size being accomplished Without loss in lens effect.

In carrying out the preceding object, I provide the respective plates with a plurality of 90 degree bends so as to produce serpentine paths through the lens between the plates, the paths being the necessary lengths required to distribute the sound waves in the desired pattern but the front-to-back overall dimension of the lens being nited States Patent Patented Aug. 19, 1958 reduced by the aforesaid bends to about .707 times the greatest serpentine path length in the axial direction.

A further important object of my invention is to provide a lens array wherein the spaces between the plates on the transverse sides of the lens need not be closed in order to contain the waves and prevent their escape through the sides of the lens. It should be apparent that if the waves are to be distributed wtihin a desired sector, substantially all of the wave energy must pass through the length of the lens rather than be permitted to escape through the sides thereof. If the sides of the lens were to be closed to prevent such losses, there would result cavity resonance conditions which would be highly undesirable. Therefore, rather than close the sides of the lens, I have extended the width of the plates beyond the width of the paths along which the waves travel through the lens.

The basic purpose of the lens is to delay the passage of the waves as they pass therethrough, different amounts of delay being introduced into different portions of the input wave front as it passes through the lens. If the input wave is to be spread horizontally, a minimum of delay should be introduced into the central portion of the wave, and an ever-increasing amount of delay should be introduced into the travel time of the wave outwardly from the center of the wave horizontally of the lens. In other words, the path length through the lens is a minimum in the center thereof and gradually increases to a maximum toward the outer sides of the lens. In order to provide the graduated delay set forth above, my lens is cut out in the center, as viewed from above, along a curve which is symmetrical on each side of the axis of the lens. The curve of the cut-out portion is laid out to provide the lens with the various serpentine path lengths required to produce the desired wave-distributing characteristics, the lens illustrated in the present disclosure being designed to provide a uniform distribution of the wave energy over a sector.

A straight portion of the lens is left on each side of the cut-out front thereof. The reason for leaving a straight portion of the lens on each side of the cut-out is to provide the increased path length necessary to act upon the outer curved portions of the lower frequency wave fronts entering. the rear of the lens in order to establish the desired output distribution of those waves. It is not necessary to introduce as much delay into the outer peripheral portions of these lower-frequency waves since the curved waves are already partially divergent in the desired direction.

Other objects and advantages of my invention will become apparent during the following discussion of the drawings, wherein:

Figure l is a perspective view of my lens looking at the front thereof from a position somewhat above and to the right of the axis of the lens.

Fig. 2 is a plan view of the lens.

Fig. 3 is a front elevation view of the lens.

Fig. 4 is an enlarged side elevation view along the line 4-4 of Fig. 2.

Fig. 5 is an enlarged section view through the lens along line 5-5 of Fig. 2.

Fig. 6 is an exploded schematic view of my lens in front of a driver unit, this figure showing two input wave fronts of different frequencies emitted by the driver unit and showing the spreading of said waves through 90 degree sectors through which the waves are uniformly distributed.

Referring now to the drawings, it will be seen that the lens comprises a plurality of plates 1 each of which plates, in the particular form illustrated, has four 90 degree bends, the bends lying transversely of the axis of the lens and the bends each being spaced from the other by an equal distance longitudinally of the lens. Thus when these plates are stacked in the manner illustrated in the drawings, the bends of each plate correspond, in vertical position, to the bends of each of the other plates, and each two adjacent plates thereby form serpentine paths, the lengths of the paths increasing outwardly from the axis of the lens in a horizontal direction. Whether the bends be 90, or some other angle, it is desirable that the surfaces of the plates between the bends lie at an angle of one half the angle of the bends with respect to the axis of the lens and transducer. The manner in which these plates are secured together is not of great importance so long as the spacings between the respective plates are identical and so long as the various plates are sufficiently rigidly secured together so as to prevent shifting of the plates, or so as to prevent undue mechanical vibration thereof. The securing means should also be small in view of the fact that the securing means actually create discontinuities in the wave patterns of the compressional waves emitted from the lens. One means for securing the plates together, as shown in the drawings, comprises a plurality of vertically disposed rods 2, said rods being welded or soldered to the plates or otherwise secured in any convenient manner so as to hold the plates rigidly to the rods at the respective points of contact thereof.

As will be seen in Fig. 2, the contour of each of the shaped plates includes a straight rear portion 1a and two straight front portions 1b, these front portions 1b being separated by a curved cut-out portion 10, the width of the cut-out portion 10 between the front portions 1b being approximately the same as the width of the transducer T and located directly in front of the latter. The sides of the lens 1d are wider than the paths followed by the input waves within the lens and are open along their outer side edges.

By examination of Figs. 2 and 5 it will be seen that at the central portion of the lens the path length through the lens is a minimum, as is illustrated at 1e in Figs. 2 and 5. However, because of the shape of the cut-out 1c the path length axially through the lens increases progressively for compressional waves which enter the back 1a of the lens horizontally offset from the central axis thereof.

Referring now to Fig. 6, the lens is shown coupled to a transducer T and, as illustrated, is spaced therefrom by a short distance so as to permit two input wave fronts to be shown, the fronts including a plane high-frequency front H and a curved lower-frequency front L. The wave front H is shown again at H1 in front of the lens as a plane wave front which would result if the front H were radiated to the position H1 in the absence of a lens. However, due to the curving action of the lens, the output wave therefrom is curved as shown at H2 and the curve H2 in this particular example subtends an angle of 90. The wave front H3 is merely a projection in the forward direction of the wave front H2, and, as in the case of the latter, subtends an angle of about 90 from the center point CH.

The wave front L is not confined to the central or cut-out portion of the lens, but also passes through the straight portions of the lens opposite the straight front portions 1b. The curvature of the front L is increased in the straight portions of the lens by the increased path length through the lens in a diagonally divergent direction with respect to the lens axis. The front shown at Ll represents the front which would result from radiation of the front L in the absence of the lens, but when the lens is present the wave front L is distributed through a wider angle and assumes the front as shown at L2. When the front L2 is projected further outwardly to coincide substantially with the front H3, a front L3 results.

Actually, the fronts L3 and H3 do not exactly coincide since they each subtend an angle of 90 but have 4 different centers. The center of the front H3 is at CH, whereas the center of the front L3 is at CL. However, it is to be noted that the lines x--x and yy are mutually parallel and that in the working model of the lens, the distances D between the lines are only about four inches, so that the discrepancy between the coverage by the wave H3 and by the wave L3 is extremely small when the waves are radiated outwardly into the area of a room.

The front H3 represents the radiated pattern for the highest frequency for which the lens is designed to operate, and the front L3 represents the lowest frequency which the lens is to act upon. Thus, the intermediate frequency fronts will lie between these limits. Actual laboratory experiments conducted on the above mentioned working model using random noise to excite the transducer and sampling the pattern within 45 either side of the axis of the lens by means of a high-grade microphone indicated that the lens was performing its function of uniformly distributing the various frequency components over the desired sector, as set forth above.

I do not limit my invention to the particular lens shown in the drawings for obviously changes may be made within the scope of the following claims.

I claim:

1. A lens for use with a compressional wave transducer emitting bot-h plane waves and arcuately divergent waves, comprising a plurality of identical parallel stacked plates mutually spaced by a constant gap and rigidly secured in superposed orientation, said plates forming a composite lens assembly symmetrical about its central axis and having a central portion between side portions and having open front, rear and sides, the rear being flat and communicating with said transducer at its forward axis and the front central portion having a curved cut-out recess of approximately the same width as said transducer and diminishing toward the rear of the assembly and said side portions being of constant length in a direction parallel with said central axis and extending outwardly between the front and the rear of the lens sufliciently that the most divergent of said waves will remain between the plates during passage from the rear to the front of the lens.

2. In a lens as set forth in claim 1, the axis of the composite lens assembly and of said recess being coincident with the axis of said transducer and each of said plates having a plurality of bends along lines transverse to said axis and equally spaced from each other, said bends permitting shortening of the front-to-back dimension of said assembly for a desired maximum compressionalwave path length.

3. In a lens as set forth in claim 2, the surfaces of said plates between said bends lying at an angle of one-half the angle of said bends with respect to said axis.

4. A lens for use with a compressional wave transducer comprising a plurality of identical parallel stacked plates mutually spaced by a constant gap and rigidly secured in superposed orientation, said plates forming a composite lens assembly having a center portion between side portions and having open front, rear and sides, the rear communicating with said transducer at its forward axis and being parallel with said front, the latter having a curved cut-out recess, said recess being broad at the front of the assembly and diminishing toward the rear of the as sembly and being symmetrical about the axis of the transducer, the width of the recess at the front of the assembly approximating the width of said transducer and the width of said assembly as measured across the front including said side portions being greater than that of said cut-out recess whereby compressional waves emitted by said transducer along paths divergent with respect to said axis will travel through the lens assembly along paths lying between said recess and said sides of the assembly.

5. In a lens as set forth in claim 4, the axis of the composite lens assembly and of said recess being coincident with the axis of said transducer and each of said plates having a plurality of bends along lines transverse to said axis and equally spaced from each other, said bends permitting shortening of the front-to-back dimension of said assembly for a desired maximum compressional-wave path length.

6. In a lens as set forth in claim 5, the surfaces of said plates between said bends lying at an angle of one-half the angle of said bends with respect to said axis.

7. In a lens for use with a compressional-wave transducer and having a central portion including serpentine paths of graduated lengths in the direction axial of said lens for distributing in space the energy emitted in plane fronts along the axis of said transducer, side portions forming continuations of said lens beyond the width of said central portion, comprising serpentine chambers offset from said central portion and communicating therewith, said chambers being of constant path length in the axial direction whereby waves emitted in convex fronts by said transducer will enter the lens along paths divergent with respect to said axis and travel diagonally outwardly through said side chambers, the path lengths diagonally of said axis being longer than said constant path length.

8. A lens for use with a compressional-wave transis; ducer comprising a central portion having serpentine paths of graduated lengths in the direction axial of said lens for distributing through a selected sector in space the Wave energy emitted in plane fronts along the axis of said transducer; and two side portions of the lens forming continuations thereof and extending beyond the Width of said central portion to provide serpentine paths of nongraduated length in the axial direction, whereby waves emitted in convex fronts by said transducer and entering the lens along paths divergent with respect to the axis Will travel paths diagonally longer than the longest axial paths.

References Cited in the file of this patent UNITED STATES PATENTS Koch July 27, 1954 OTHER REFERENCES 

